1. Field of the Invention
The present invention relates generally to communications systems. More specifically, the present invention relates to a communication system in which an information-modulated electromagnetic wave has a carrier frequency and an electric field corresponding to a rotation vector tracing a nonlinear predictable path at a second frequency that is less than the carrier frequency of the wave.
2. Description of the Related Art
An electromagnetic wave can be defined by an electric field and a magnetic field that are orthogonal to one another along an axis of propagation. The behavior of the wave can be described with respect to the orientation of the field vector of the electric (E) field.
Polarization is a term that can be used to characterize the orientation of the field vector of an E field of some electromagnetic waves. Different types of polarization include: linear (also referred to as plane), circular, and elliptical polarization.
Where the field vector of an E field propagates within a plane as the wave propagates along an axis, the polarization of the wave is referred to as linear or plane polarization. Where the terminus of the E field, i.e., the extremity of the field vector, in a given plane perpendicular to the axis of propagation traces a circular path rotating about the axis of propagation at a frequency equal to the frequency of the wave, the polarization is referred to as circular polarization. Similarly, where the terminus of the E field, in a given plane perpendicular to the axis of propagation, traces an elliptical path rotating about the axis of propagation at a frequency equal to the frequency of the wave, the polarization is referred to as elliptical polarization, a general case of circular polarization.
Polarized waves can be transmitted or received in a number of different ways. For example, an antenna itself can impose a certain polarization upon a transmitted wave or be sensitive to received waves of a certain polarization. A dipole antenna oriented horizontally with respect to the earth is suited to receive and/or transmit linearly polarized waves where the plane of polarization is parallel to the earth. Similarly, a dipole antenna oriented vertically with respect to the earth is suited to receive and/or transmit linearly polarized waves where the plane of polarization is perpendicular to the earth""s surface. A helical antenna is suited to receive and/or transmit circularly polarized waves.
Communications systems transmitting and receiving polarized waves can be adversely affected by apparent prolonged fading of transmitted/received waves having only one type of polarization. To minimize fading of the amplitude of the received wave having the one type of polarization, communication systems can be designed to transmit and receive multiple waves each having a different polarization. This method can be characterized as polarization diversity.
Polarization has also been used to avoid inter-channel interference in, for example, satellite communications systems. A satellite can communicate with a ground station using right-hand (i.e., clockwise (CW)) circular polarized waves at a given carrier frequency, while an adjacent satellite can communicate with another ground station at the same carrier frequency using left-hand (i.e., counterclockwise (CCW)) circular polarized waves. Helical antennas having opposite twists can be used to receive and/or transmit left-hand and right-hand circular polarized waves.
Polarization can be used to encode information in a communications system. U.S. Pat. No. 4,084,137, issued to Welti, describes a communications system that encodes a horizontally polarized wave and a vertically polarized wave in accordance with information. U.S. Statutory Invention Registration H484 describes a similar system that addresses a sidelobe problem in a radar system.
The polarization encoding concept addressed in the references described above can also be used to minimize the likelihood of unauthorized interception of a message. U.S. Pat. No. 5,592,177, issued to Barrett, describes a communications system that sequentially changes the polarization of a signal-carrying wave in a pseudorandom manner. The Barrett system provides broad polarization bandwidth for transmitting and/or receiving signals while minimizing the required frequency bandwidth of the transmitter and receiver systems. The selected polarizations include linear polarization with a variable polarization plane orientation, right-hand and left-hand circular polarizations, and right-hand and left-hand elliptical polarizations with a variable ellipse major axis orientation. By changing the specific polarization, the signal is spread in polarization in a manner analogous to the spreading of a signal over a continuous range of frequencies in spread spectrum communications systems. Note that when the signal-carrying wave is circularly or elliptically polarized, the field vector of the generated E field is rotating a frequency equal to the carrier frequency.
The concept of transmitting separately encoded horizontally polarized waves and encoded vertically polarized waves can also be used for channel discrimination in a two-channel communications system in which the channels have the same carrier frequency. U.S. Pat. No. 4,521,878, issued to Toyonaga, describes a communications system that encodes a horizontally polarized wave and a vertically polarized wave in accordance with a first code to form a signal corresponding to a first channel, and encodes a horizontally polarized wave and a vertically polarized wave in accordance with a second code to form a signal corresponding to a second channel. The system thus improves cross-polarization discrimination over known systems that attempt to simply transmit a first channel using horizontal polarization and a second channel using vertical polarization.
These known communications systems, however, suffer shortcomings. Regardless of the type of polarization used by the known communications systems, the E-field vector of an electromagnetic wave is either linearly polarized or elliptically polarized and consequently rotates about the axis of propagation at a frequency that equals the carrier frequency of the wave.
The present invention can increase the amount of information carried by a communications system for a given carrier frequency. The present invention increases the amount of information carried by a communications system within a discrete carrier in an appropriate medium by producing more than one information channel for each carrier frequency within the frequency allocation.
The selectivity of the present invention results in lower noise and therefore creates a higher signal-to-noise ratio for an information channel. The present invention provides an information channel where the noise is limited to characteristics of the channel.
The present invention relates to a communications system in which a communications channel is defined at least in part by an electromagnetic wave having a carrier frequency and an electric (E) field vector the extremity of which traces a non-linear periodic path (or a predictable path whose rate of change about the axis of propagation is at a frequency less than the carrier frequency) at a second frequency less than the carrier frequency from the perspective of an observer located at a plane perpendicular to the axis of propagation of the wave. The transmitter of the communications system creates an electromagnetic wave having such characteristics and which is modulated with information in a suitable manner. The receiver of the communications system is sensitive to the periodic path and carrier frequency of the E-field vector. The combination of E-field vector path and the carrier frequency provides selectivity that can be used to define a communications channel.
In certain exemplary embodiments of the invention, a communications channel is defined at least in part by an electromagnetic wave with an E-field vector, as projected onto or from the perspective of a plane transverse to the axis of propagation, rotating at a selected angular velocity that is less than and independent of the carrier frequency. The transmitter and receiver of the system are each synchronized to a rotation frequency that defines the angular velocity.
Although in certain embodiments the E-field vector can remain at a selected second frequency for an indefinite time interval, such as that which is sufficient to communicate an entire message, in other embodiments the E-field vector can change from one second frequency to another at any suitable predictable manner whereby the system can communicate some amount of information between changes, however large or small that amount of information may be. Frequency hopping and sequencing constitute a class of communications techniques that can readily be applied to the present invention in view of these teachings whether to the carrier frequency or to the rate of rotation of the E field vector about the axis of propagation.
In another embodiment of the present invention, a transmitter using a single carrier frequency produces a wave having an E-field vector that rotates at an angular velocity less than the carrier frequency. The transmitter can produce the wave by providing a rotation frequency signal source, an antenna system having two or more elements, and two or more phase systems, each corresponding to one of the antenna elements. In such an embodiment, each phase system includes a suitable time-delay, such as a delay line or a phase shifter, that delays the rotation frequency signal by a fixed amount such that the sum of the delays becomes a constant value. Each phase system also includes a suitable amplitude modulator, such as a voltage-variable attenuator, a balanced modulator, or other device, that amplitude modulates the information-modulated carrier signal with the time-delayed rotation frequency signal. Each antenna element receives the amplitude-modulated output of one of the phase systems. In one embodiment of the present invention, the antenna elements are dipoles aligned at different angular orientations.
In another embodiment of the present invention, a receiver using a single carrier frequency can recover the information signal from a wave having an E-field vector rotating at an angular velocity less than the carrier frequency. The receiver can recover the information signal by providing a rotation frequency signal source, an antenna system having two or more elements, two or more phase systems, each corresponding to one of the antenna elements, and a combiner. Although the wave impinges upon each antenna element, each antenna element produces a corresponding received signal that represents only one projection component of the wave. Each phase system is essentially the inverse of that provided in the transmitter described above. As in the transmitter, each phase system includes a suitable time-delay, such as a delay line or a phase shifter, that delays the rotation frequency signal by a different but known amount. Each phase system also includes a suitable amplitude modulator, such as a voltage-variable attenuator, a balanced modulator, a single balanced mixer, a double balanced mixer, or other device, that gates the received signal provided by the corresponding antenna element in accordance with the time-delayed rotation frequency signal. Because the rotation frequency signal defines a channel characteristic, signals outside the channel are attenuated. The combiner sums the detected amplitude-modulated signals produced by the phase systems. In one embodiment of the present invention, the antenna elements are dipoles aligned at different angular orientations.
In another embodiment of the present invention, a transmitter sends two waves each having a different carrier frequency and having opposite circular polarizations to produce a resulting superposed wave having its own carrier frequency and an E-field vector that rotates about the propagation axis at a frequency less than the new carrier frequency. The transmitter can produce the wave by providing a compound antenna system, a lower differential carrier frequency source, an upper differential carrier frequency source, and two synchronized amplitude modulators. The upper and lower differential carrier frequency sources produce upper and lower differential signals, respectively. The upper differential signal has a frequency equal to the carrier frequency plus the rotation frequency, and the lower differential signal has a frequency equal to the carrier frequency minus the rotation frequency. The average of the differential signals corresponds to the new carrier frequency of resultant wave. One of the amplitude modulators modulates the upper differential signal with an information signal, and the other modulates the lower differential signal with the same information signal. Each of the information-modulated differential signals is coupled to one of the antenna elements.
In one embodiment of the present invention, the antenna system includes a compound antenna having two helical antenna elements each producing waves with E-field vectors rotating about the axis of propagation in opposite directions. The antenna element driven by the differential carrier signal having the higher of the two frequencies dictates the direction of rotation about the propagation axis of the E-field vector of the resulting wave. The E-field vector of the resulting wave rotates about the propagation axis in a clockwise direction if the antenna element having a clockwise twist is driven by the upper differential signal, and the antenna element having a counterclockwise twist is driven by the lower differential signal. The E-field vector of the resulting wave rotates about the propagation axis in a counterclockwise direction if the antenna element having a counterclockwise twist is driven by the upper differential signal, and the antenna element having a clockwise twist is driven by the lower differential signal.
In another embodiment of the present invention, a receiver tuned to two differential carrier frequencies can recover the information signal from a wave having an E-field vector rotating about the propagation axis at a rotation frequency less than the average of the two carrier frequencies. The receiver can recover the information signal by providing two filters, one coupled to one antenna element and the other coupled to another antenna element of a dual antenna system, a summing circuit coupled to the filters for summing the received upper and lower differential signals, and an amplitude modulation detector circuit coupled to the output of the summing circuit. One filter has a passband centered around the lower differential frequency, and the other has a passband centered around the upper differential frequency.
In one embodiment of the receiver, the antenna system includes a compound antenna having two helical antenna elements each receiving wave components of the resultant wave: received wave components have E-field vectors rotating about the propagation axis in opposite directions and superpose to the resultant wave. Each received wave component corresponds to the information-modulated differential signals sent by the transmitter.
Note that the transmitted carrier has no effective sidebands from the perspective of the resulting channel; the term xe2x80x9csidebandxe2x80x9d is used herein only for convenience with respect to certain embodiments. The term merely evokes the concept that the transmitted signal would have sidebands from the perspective of the resulting channel were it not for the summation of the energy radiated by the antenna elements of the antenna system. The quadrature summation cancels the frequencies that a single antenna element would radiate in the absence of the other antenna elements.
In the embodiments of the present invention in which the extremity of the E-field vector of the electromagnetic wave rotates at a second frequency that is less than the carrier frequency, the quantity E curl H of the propagating wave remains constant, where E is the electric field vector and H is the magnetic field vector, when the wave is not modulated with information. The quantity E curl H represents the total energy of the field. Of course, when the wave is modulated with information, the quantity E curl H of the wave no longer remains constant.
The communication system of the present invention may be used in any suitable dielectric medium that supports oriented electromagnetic waves, such as air, free space, waveguides, and optical fiber.
Although the embodiments described above relate to a communications system in which a communications channel is defined by a selected E-field vector rotation frequency of a electromagnetic wave that is less than the carrier frequency of the wave, more generally, the invention relates to a system in which a communications channel is defined at least in part by a wave having a carrier frequency and an E-field vector, the extremity of which traces a non-linear periodic path at a second frequency (i.e., a rotation frequency) less than the carrier frequency from the perspective of an observer located at a plane perpendicular to the axis of propagation of the wave. Thus, the path the extremity of the E-field vector may trace is not limited to a regular path as described above that results from rotation of the E-field. Rather, it may include other non-linear paths that are more irregular. In the general case, the path of the extremity of the E-field vector traces a predictable path where the frequency of the change of path is less than the carrier frequency. For example, the path may be defined by a pseudorandom sequence generator. Essentially any non-linear periodic path (or more generally, any non-linear path) that both a transmitter and receiver of a communications system can follow in synchronism, for example, at a rotation frequency less than the carrier frequency, would be suitable.
A Poincare sphere is a graphical representation that comparatively illustrates polarizations. The poles of the sphere represent right-hand and left-hand circular polarizations. Points on the equator represent linear polarizations of various orientations with respect to horizontal and vertical. Points on one hemisphere represent various right-hand elliptical polarizations, and points on the other hemisphere represent various left-hand elliptical polarizations. A conventional Poincare sphere is not sufficient to describe the E-field vector path of waves that behave in accordance with the present invention because a Poincare sphere describes only waves having conventional circular, elliptical and linear polarizations, i.e., waves having E-field vector extremities that follow circular, elliptical and linear periodic paths at a frequency equal to the carrier frequency of the wave.
Nevertheless, in embodiments of the present invention where the E field rotates from the prospective of a plane perpendicular to the propagation axis, if one considers a Poincare sphere in a novel or modified manner such that its radius corresponds to the carrier frequency, then the interior of the sphere describes waves that behave in accordance with to certain embodiments of the present invention. (The interior of a conventional Poincare sphere has no meaning in the prior art; only the surface is relevant.) Points near the center of such a modified Poincare sphere would describe waves having E-field vectors rotating at a frequency near zero. Points on any spherical radial axis extending between the center and the surface of the sphere describe waves that behave in accordance with the present invention. In particular, the points on the polar axis between the center and the poles would describe waves having an E-field vector that rotates at a rotation frequency less than the carrier frequency as described above with respect to certain embodiments. Each point or interval on the polar axis could be used to define a discrete communications channel.
The wave may be modulated with information in any suitable manner. Although as described above the carrier frequency is amplitude modulated with the information, it is believed in accordance with the present invention that the carrier frequency may be frequency modulated with information or modulated with the information in any other suitable manner. For example, the second frequency (i.e., the rotation frequency) at which the extremity of the E-field vector traces the path may be modulated with information. Where the extremity of the E-field vector traces a path at a modulated second frequency (i.e., a modulated rotation frequency), the deviation of the wave from the basic rotation frequency (i.e., the non-information-modulated rotation frequency) represents the information in a manner analogous to that in which the deviation of a conventional frequency modulated signal from a center channel frequency represents the information.
The foregoing, together with other features and advantages of the present invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.